It has been a fundamental tenet of nutrition: When it comes to weight loss, all calories are created equal. Regardless of what you eat, the key is to track your calories and burn more than you consume.
But a large new study published on Wednesday in the journal BMJ challenges the conventional wisdom. It found that overweight adults who cut carbohydrates from their diets and replaced them with fat sharply increased their metabolisms. After five months on the diet, their bodies burned roughly 250 calories more per day than people who ate a high-carb, low-fat diet, suggesting that restricting carb intake could help people maintain their weight loss more easily.
The new research is unlikely to end the decades-long debate over the best diet for weight loss. But it provides strong new evidence that all calories are not metabolically alike to the body. And it suggests that the popular advice on weight loss promoted by health authorities — count calories, reduce portion sizes and lower your fat intake — might be outdated.
“This study confirms that, remarkably, diets higher in starch and sugar change the body’s burn rate after weight loss, lowering metabolism,” said Dr. Dariush Mozaffarian, the dean of the Friedman School of Nutrition Science and Policy at Tufts University, who was not involved in the research. “The observed metabolic difference was large, more than enough to explain the yo-yo effect so often experienced by people trying to lose weight.”
Dr. Mozaffarian called the findings “profound” and said they contradicted the conventional wisdom on calorie counting. “It’s time to shift guidelines, government policy and industry priorities away from calories and low-fat and toward better diet quality.”
The new study is unique in part because of its size and rigor. It is among the largest and most expensive feeding trials ever conducted on the subject. The researchers recruited 164 adults and fed them all of their daily meals and snacks for 20 weeks, while closely tracking their body weight and a number of biological measures. The trial cost $12 million and was supported largely by a grant from the Nutrition Science Initiative, a nonprofit research group co-founded by Gary Taubes, a science and health journalist and proponent of low-carbohydrate diets. The study was also supported by funding from the New Balance Foundation, the National Institutes of Health and others.
While some experts praised the findings, others were more cautious. Dr. Kevin Hall, a scientist and obesity expert at the National Institute of Diabetes and Digestive and Kidney Diseases, said the new study was ambitious and very well run. But he said the researchers used methods that raise questions about the results. One method they used to track metabolism, called doubly labeled water, has not been shown to be reliable in people on low-carb diets and it may have exaggerated the amount of calories the subjects burned, he said.
Dr. David Ludwig, an endocrinologist at Harvard Medical School and Boston Children’s Hospital and one of the study authors, disagreed, saying: “We used a gold standard method that has been validated across a wide range of experimental conditions and universally adopted in the field.”
Dr. Hall added, “I would love it to be true that there was a diet combination of carbs and fats that led to large increases in energy expenditure — and I really hope it is true. But I think there are reasons to question whether or not it is.”
The idea that counting calories is the key to weight loss has long been embedded in the government’s dietary guidelines. It is the driving force behind public health policies like mandatory calorie counts on restaurant menus and food labels. Many experts say that the underlying cause of the obesity epidemic is that Americans eat too many calories of all kinds, prompted by easy access to cheap and highly palatable foods, and that they need to exercise portion control. On its website, for example, the National Institutes of Health encourages people to count calories and warns that dietary fat has more calories per gram than protein or carbs: “You need to limit fats to avoid extra calories,” it states.
But experts like Dr. Ludwig, argue that the obesity epidemic is driven by refined carbohydrates such as sugar, juices, bagels, white bread, pasta and heavily processed cereals. These foods tend to spike blood sugar and insulin, a hormone that promotes fat storage, and they can increase appetite. Dr. Ludwig and his colleague Dr. Cara Ebbeling have published studies suggesting that diets with different ratios of carbs and fat but identical amounts of calories have very different effects on hormones, hunger and metabolism. He has also written a best-selling book on lower-carb diets.
Dr. Hall and others disagree. They have published studies disputing the notion that carb-restricted diets accelerate metabolism and fat loss. Dr. Hall said that low-carb diets have many benefits: They can help people with Type 2 diabetes manage their blood sugar levels, for example. But he argues that the carb and insulin explanation for obesity is too simplistic and has been “experimentally falsified” in rigorous studies.
Dr. Hall published a meta-analysis of feeding studies last year that suggested that energy expenditure was actually slightly greater on low-fat diets. But Dr. Ludwig pointed out that those studies were very short, with none lasting longer than a month and most lasting a week or less. He said the process of adapting to a low-carb diet can take a month or longer.
“A few days, or a couple weeks, is not enough time to make any meaningful conclusion about how diets affect metabolism over the long term,” he added.
To do the new study, Dr. Ludwig and his colleagues collaborated with Framingham State University, about 20 miles outside of Boston, where they recruited overweight students, staff members and faculty members. Each participant went through two phases of the study. First, they were put on strict diets that lowered their body weight by about 12 percent, which was designed to stress their metabolisms.
“At that point their bodies are trying to regain the weight,” Dr. Ludwig said. “It pushes the body and predisposes to weight regain.”
In the second phase of the study, the subjects were assigned to one of three diets with 20 percent, 40 percent or 60 percent of their calories from carbohydrates. Protein was kept steady at 20 percent of calories in each diet.
Over the next five months, the researchers tracked the subjects meticulously and provided them with enough daily meals and snacks to keep them from losing or gaining any weight. This was so the researchers could determine precisely how the subjects’ metabolisms responded to the different diets while their body weight remained stable.
The researchers tracked biomarkers that helped them ensure that the participants stuck to their diets. They also worked with a large food service company, Sodexo, to prepare thousands of generally healthful meals that the subjects could eat in cafeterias or take home with them. A typical meal for the high-carb group might consist of a chicken burrito bowl with rice and vegetables, for example, or roasted turkey with green beans and mashed potatoes. The low-carb group would get a similar meal with fewer carbohydrates, like a chicken burrito lettuce wrap or roasted turkey with green beans and mashed cauliflower.
What the researchers found was striking. The roughly 250 extra calories that the subjects in the low-carb group burned each day could potentially produce a 20-pound weight loss after three years on the diet, Dr. Ludwig said. People who tended to secrete higher levels of insulin did the best on the low-carb diet, burning about 400 extra calories a day.
The subjects on the low-carb diet also had the sharpest declines in a hormone called ghrelin, which is produced in the stomach. Ghrelin promotes hunger and body fat, and it lowers energy expenditure. Suppressing ghrelin may be one reason the low-carb diet increased metabolism, the authors noted.
Dr. Ludwig emphasized that the results need to be replicated by other investigators and he stressed that the findings do not impugn whole fruits, beans and other unprocessed carbohydrates. Rather, he said, the study suggests that reducing foods with added sugar, flour and other refined carbohydrates could help people maintain weight loss by increasing their metabolisms at a lower body weight.
“These foods seem to undermine your metabolism,” he said. “They slow metabolism in a way that may work against long-term weight loss maintenance.”
